1. Field of Invention
This invention relates generally to air purifiers for use in the home or in the office for removing particulate contaminants from the atmosphere; and more particularly to a unit of this type which combines mechanical and electrostatic filtration to ensure the removal of both fine and coarse particles and which at the same time imparts a pleasing aroma to the atmosphere.
2. Status of Art
The typical modern urban and suburban environment is almost never pure and salubrious. The degree of pollution which permeates the atmosphere depends on whether the city is a site for so-called smoke-stack industries which spew contaminants into the atmosphere, and also on whether local electric power plants are coal-burning. Other factors are the presence of incinerators and the density of vehicular traffic. Automotive and truck exhausts are a major source of pollutants in many communities.
Where the city is notoriously windy as is in the case for Chicago, or is somewhat less so as in New York, many contaminants are blown away by the prevailing winds, or at least transferred to other communities. In the case of a city such as Los Angeles which is not heavily industrialized, the pollution level may nevertheless be high because of static air conditions.
In some American cities, environmental tests are conducted on a day to day basis, and when the measured pollution level is so high as to represent a danger to those suffering from pulmonary disorders or allergies which are adversely affected by pollutants, local residents are then warned to stay indoors until a clear signal is sounded. This practice is based on the supposition that the indoor environment is somewhat less polluted. But recent studies conducted by environmental agencies indicate that in many homes the indoor atmosphere is no less polluted than that outside.
The reason for this surprising conclusion lies in the efficiency of modern home insulation and the absence of drafts. A well insulated home is almost hermetically isolated from the exterior atmosphere and therefore undergoes a gradual build up of contaminants which are internally generated. These contaminants have many sources, such as air-borne fibers derived from rugs and other fabrics, hairs from pet animals and particulate matter thrown off by household ovens and furnaces. As a consequence, little may be gained when one is secluded in a home to avoid outdoor pollution, for the interior atmosphere may not be free of contaminants.
Various domestic filtration units are commercially available for home use. Thus the typical household air conditioner includes a fibrous or open cell foam plastic filter to remove coarse particles from air passing through the conditioner. Some electric ovens now on the market include a high voltage electrostatic precipitator unit to purify the air in the region of the oven.
U.S. Pat. No. 3,315,445 to deSeversky points out that mechanical filtration systems such as centrifugal and cyclonic collectors which function to remove contaminants from an air stream are only effective for particles of over 25 microns in size. Much finer particles cannot be extracted by conventional mechanical means. The most effective way of removing very fine particles is by electrostatic precipitation in which the particles in the stream are negatively ionized and conveyed through a positively-charged tubular collector.
In the resultant electrostatic field, the negatively charged particles are caused to migrate toward the collector so that the air stream emerging from the collector is substantially free of particles. In the system disclosed in the deSeversky patent, the air stream is first forced through a scrubber and then passes through an electrostatic precipitator. This system is suitable for industrial installations but is inappropriate for home use.
In purifying the air in the home it is essential that very fine as well as relatively coarse particles be removed. Fine particles tend to pass through the natural filters existing in nasal passages and penetrate the pulmonary system to a greater degree than coarse particles. Thus a familiar objection to the typical fibrous filter placed at the mouth end of a cigarette is that it acts only to remove coarse particles from the smoke and offers little protection against finer particles which are inhaled into the lungs by the smoker.
In the home or office, it is not only essential to remove contaminants from the air to render the atmosphere free of irritants, but it may also be desirable to impart a pleasing fragrance thereto to mask unpleasant odors. The use of aroma generators or air fresheners for this purpose is commonplace, but such devices have heretofore been in the form of separate units.